1. FIELD OF THE INVENTION
The present invention relates to a thermal fuel level detector capable of detecting the level of fuel remaining in a fuel tank of an automobile or the like.
2. RELATED ART STATEMENT
Previously, most fuel level detectors of the type described above comprises a slide-content type potentiometer attached to an end of a fuel level float. A conventional structure in which such a potentiometer is employed experiences a problem of contact damage therein due to the presence of sulfides or other fuel additives since the contact thereof is exposed to fuel or steam. It is difficult to keep the conventional structure providing accurate slide-resistance values for a long time.
Another problem is experienced with the conventional structure in that it is difficult to obtain a resistor with a shape which corresponds to the shape of a fuel tank, causing a large error in detection of a fuel level.
Although various methods have been devised in order to overcome the above-described problems, none has yet been put into practice due to cost and performance drawbacks. For example, a liquid level sensor disclosed in Japanese Patent Laid-Open No. 59-148826 and structured such that thermistor layers are formed in order on a rod-like or an elongated insulating material can relatively accurately detect the liquid level under certain temperature conditions. However, since thermistors do not exhibit linear resistance changes with respect to temperature changes, such liquid level sensors cannot accurately detect a continuous change in liquid level where the temperature of either the liquid or the ambient changes even if a temperature compensating sensor is additionally provided. Thermistors further experience a problem of insufficient protection against gasoline containing sulfides, more particularly against light oil. Therefore, such liquid level sensors cannot be used as a fuel level detector for an automobile.
Furthermore, the Nippon Denso Publication Technology Monogram (No. 48-101, published on July 15, 1986) discloses a system formed, as shown in FIG. 1, such that: one of two metal wires made of the same material and having the same temperature resistance coefficient is arranged to be a self-heating sensing resistance wire 40, with the other of the wires being arranged to be a temperature compensating resistance wire 41; the self-heating sensing resistance wire 40 is in part dipped in liquid fuel; external resistors 42 and 43 whose resistance values are equal are used with the former to form a bridge circuit; a potential difference between junctions 44 and 45 is amplified by a differential amplifier 46 to detect a difference in temperature between the self-heating resistance wires corresponding to said potential difference, so that the liquid level is detected. However, this system involves two serious problems. First, since the temperature compensating resistance wire 41 is disposed in the part of the fuel tank in which the same is not dipped in the liquid fuel, although a correction to compensate a change in ambient temperature, a correction cannot be made to compensate a change in temperature of the liquid fuel. Secondly, since metal wires are used, the resistance value per unit length of the same becomes excessively low even if the diameter of the wire is made as small as possible while still ensuring the strength of the metal wire. This leads to a problem in that the power consumption by the above-described system imposes a high load on a car battery which is used as the power source of this system. Therefore, such a system cannot be put into practical use.